Adipose tissue development is a crucial contributor to obesity and the associated type II diabetes and Metabolic Syndrome. Much has been learned about adipogenesis in tissue culture models, but relatively little is known about its in vivo mechanisms. Our developmental genetic studies of the nuclear receptor PPAR? - a key regulator of adipocyte differentiation and function - are revealing new principles of adipose tissue development, and the proposed project is designed to determine the mechanistic basis of these principles. We previously demonstrated that Pparg deficiency causes complete lipoatrophy, which occurs despite the formation of Pparg-null adipose primordia, and results from their failure to expand and differentiate. We show here that in chimeric embryos, which contain a mixture of wild type (wt) and Pparg-null cells, developmentally arrested Pparg-null adipose primordia are infiltrated and reconstituted by rapidly proliferating wt adipocytes, such that all chimeric mice eventually possess normal adipose tissue. Surprisingly, growth-arrested Pparg-null preadipocytes appear to resume expansion in the immediate vicinity of infiltrating wt adipocytes. However, the eventual absence of Pparg-null preadipocytes from postnatal fat pads suggests that, despite initial proliferation, Pparg-null preadipocytes likely fail to survive in the long term. In vitro, the importance of PPAR? for the early phase of adipocyte differentiation is evident in the vast improvement of mouse embryonic fibroblast (MEF) adipogenesis when cultures are treated with the PPAR? agonist rosiglitazone (rosi) for the first 72 hours of differentiation. Moreover, rosi more than doubles the number of MEF that replicate in response to adipogenic stimuli. The role of cell-cell interactions in this process is evident in a surprising inhibition of adipogenesis of 3T3-L1 cells by co-cultured MEF, and reversal of this inhibition by administering rosi during the first 72 hrs of differentiation. A microarray-based screen for potential PPAR? targets that may transduce these early adipogenic functions identified several novel genes related to Wnt signaling, cell death, and cell growth. These data generate the hypotheses that PPAR? controls mitotic expansion of preadipocytes and that it regulates this process via non cell-autonomous effectors. The specific aims will test these hypotheses and generate crucial new insights into adipose tissue development by determining the mechanisms of PPAR? action during early adipogenesis in vivo and in vitro. Specific Aim 1 will use chimeric embryos to dissect the cell-autonomous and non cell-autonomous functions of PPAR? in adipocyte proliferation, survival, and differentiation in vivo. Specific Aim 2 will use genetic and pharmacological manipulations to dissect the functions of PPAR? and several of its novel targets in early adipogenesis in vitro. Specific Aim 3 will dissect the role of PPAR?-regulated cell-cell signaling in early adipogenesis by analyzing co-cultures of Pparg-null MEF and 3T3-L1 cells. PUBLIC HEALTH RELEVANCE: Obesity is a major cause of type II diabetes and the metabolic syndrome, and the control of adiposity is top priority in managing these diseases. Adipocyte differentiation is a major contributor to adiposity, and the proposed studies will provide critical new insights into molecular and cellular mechanisms of adipose tissue development. These insights will provide broad rationales for prevention and treatment of obesity.